1. Field of the Invention
The present general inventive concept relates to encoding and decoding of an audio signal, and more particularly, to a method and apparatus to encode/decode a low bit-rate audio signal.
2. Description of the Related Art
In the existing MPEG-4 advanced audio coding (AAC) algorithm, a full-band audio signal is encoded using a quantizing and coding method. However, at a low bit rate, a sub-band audio signal is generally encoded, because the number of available bits is small. In this case, since a bandwidth of the audio signal is reduced, poor sound quality results.
A high frequency component can be encoded only by detecting an envelope of a spectrum rather than a fine structure of the signal. Accordingly, in the MPEG-4 advanced audio coding (AAC) algorithm, a high frequency component having a strong noise component is encoded using a perceptual noise substitution (PNS) tool. For PNS encoding, an encoder detects an envelope of noise from the high frequency component and a decoder inserts random noise into the high frequency component, and restores the high frequency component. The high frequency component including stationary random noise can be efficiently encoded using the PNS tool. However, if the high frequency component includes transient noise and is encoded by the PNS tool, metallic noise or buzz noise occurs.
In an attempt to solve this problem, in the MPEG-4 high efficiency (HE) AAC algorithm, the high frequency component is encoded using a spectral band replication (SBR) tool. Since the SBR tool uses a quadrature mirror filter (QMF), in the core AAC, a modified discrete cosine transform (MDCT) output is subjected to the QMF to obtain the high frequency component. In this case, complexity increases. Furthermore, a low frequency component of a specific band is replicated and is encoded to be similar to an original high frequency signal using envelope/noise floor/time-frequency grid. However, additional information such as the envelope/noise floor/time-frequency grid requires bit rates of several kbps (kilobits per second) and a large amount of calculation.